In a conventional image forming apparatus, there is an example in which the remaining amount of the toner in a toner container is detected by using a piezoelectric sensor or an ultrasonic sensor. For example, in a remaining toner amount detecting device described in Japanese Laid-Open Patent Application (JP-A) Hei 1-6986, during rotation of an agitator, the piezoelectric sensor having a detecting portion directed upward is provided on a bottom of a hopper at a position where a thin plate-like member provided at an end portion of the agitator passes in proximity to the sensor. Further, the remaining toner amount is detected from a time ratio between a time required for one rotation the agitator and a time for which the piezoelectric sensor detects pressure by the thin photo-like member. In this remaining toner amount detecting device, in the case where the remaining toner amount is not less than a certain amount, an output of the piezoelectric sensor is fixed to logic of the presence of the toner, and when the remaining toner amount is not more than the certain amount, the output of the piezoelectric sensor is fixed to logic of the absence of the toner.
However, in JP-A Hei 1-6986, there was the following problem. That is, when the remaining toner amount is large, a time for which the weight of the toner is not detected is not generated and therefore the remaining toner amount cannot be detected until the toner amount is decreased to the certain amount. Further, with speed-up of the image forming apparatus in recent years, when a stirring member is operated at high speed, the toner in a toner container is stirred up to result in a state in which the toner is present at a detection position of the piezoelectric sensor and therefore it is difficult to ensure the time for which the weight of the toner is not detected.
Further, in another conventional image forming apparatus, a permeability sensor is used in a device for detecting the amount of the toner (developer) in a developing unit. As an example of the device for detecting the amount of the developer by using the permeability sensor, e.g., there is the detecting device as disclosed in JP-A 2002-132036. JP-A 2002-132036 discloses the toner amount detecting device which uses a flexible first stirring blade deformed toward a rear side with respect to a rotational direction by stirring the toner, a rigid second stirring blade provided at the rear side of the first stirring blade with respect to the rotational direction, and the permeability sensor provided outside the bottom of the developing unit. This device detects a state of a rotating operation of a metal material provided on each of the stirring blades by the permeability sensor provided outside the bottom of the developing unit. Further, this device is constituted so that in the case where the toner amount in the developing unit is large, the first stirring blade and the second stirring blade integrally perform the rotating operation and so that in the case where the toner amount in the developing unit is small, the first stirring blade and the second stirring blade separately perform the rotating operation without being deformed. In this case, when the toner amount is detected by using the permeability sensor, a change in permeability per rotation of a rotation shaft is detected once in the case where the toner amount in the developing unit is large and is detected twice in the case where the toner amount in the developing unit is small. The toner amount detecting device detects the toner amount in the developing unit on the basis of the change in number of this detection.
However, the detecting device of JP-A 2002-132036 involves the following problem. In the case where the first and second stirring blades integrally perform the rotating operation and therefore a signal detected by the permeability sensor indicates one change of the permeability per rotation of the rotation shaft. On the other hand, in the case where the toner amount is small, the first stirring blade is little deformed and thus the first and second stirring blade do not integrally perform the rotating operation. At this time, the signal detected by the permeability sensor indicates two changes of the permeability per rotation of the rotation shaft. In this case, selective detection of the amount of the toner or the presence/absence of the toner is made depending on the number (once or twice) of the change in magnetic field detected by the permeability sensor. For this reason, it is difficult to detect the change in toner amount in real time.